01. Gene therapy Boulikas.pdf - Gene therapy & Molecular Biology

Gene Therapy and Molecular Biology Vol 1, page 55
Figure 20. A summary of the apoptotic and cell cycle restrictive activities of p53. From Boulikas T (1997) Gene therapy of prostate
cancer: p53, suicidal genes, and other targets. Anticancer Res 17, 1471-1506. With the kind permission of Anticancer Research.
biophysical studies indicate that p53 exists as a tetramer in
solution (Stenger et al., 1992).
Increased levels of p53 upregulate the expression of
specific genes including Cip-1/Waf-1/p21 (El-Deiry et al,
1993), GADD45 (Kastan et al, 1992), cyclin G (Okamoto
and Beach, 1994), and mdm2 (Perry et al, 1993; Barak et
al, 1993; Momand et al, 1992) which is induced by UV
damage in a p53-dependent pathway (Perry et al, 1993).
Gadd45 inhibits cell cycle progression (Papathanasiou et
al, 1991).
Mdm2 acts as a feedback loop for the biological
functions of p53 apparently to moderate the G1/S arrest or
apoptosis triggered by p53 following severe damage to
DNA. Mdm2 protein associates with p53 causing p53
inactivation by preventing its sequence-specific binding to
regulatory targets in DNA (Momand et al, 1992; Oliner et
al, 1992). Elevated levels of Mdm2 mimic the effect of T
antigen, E1B of adenovirus, E6 of HPV, which also
inactivate p53 in a similar manner; overexpression of
Mdm2 can block the induction of apoptosis by p53 (Chen
et al, 1994).
Additional genes up-regulated by p53 include human
PCNA (Shivakumar et al, 1995), mouse muscle creatine
kinase MCK (Zambetti et al, 1992), EGFR (Deb et al,
1994), the potent promoter of the death pathway Bax
(Miyashita and Reed, 1995), and thrombospondin-1
(Dameron et al, 1994). Other cellular regulatory regions
that interact with p53 include the RGC repeats in the
ribosomal gene cluster (Farmer et al, 1992; Kern et al,
1992).
The PCNA promoter is up-regulated in the presence of
moderate amounts of wt p53; however, at higher levels of
wt p53 the PCNA promoter is inhibited whereas tumorderived
p53 mutants activate the PCNA promoter
(Shivakumar et al, 1995); it has been suggested that the
moderate elevation in wt p53 seen after DNA damage
induces PCNA to cope with its DNA repair activities
(Shivakumar et al, 1995); this inhibition in DNA
replication but stimulation in repair by p53 might be
accomplished by an independent pathway involving
induction of p21 (El-Deiry et al, 1993) which interacts
with PCNA protein auxiliary to DNA polymerase δ to
inhibit the replication but not the repair functions of
PCNA (Li et al, 1994).
The bax gene which induces apoptosis (Figure 21) is
upregulated by p53 whereas the bcl-2 gene which inhibits
apoptosis in B cells is down-regulated by p53 (Miyashita
et al, 1994a,b; Miyashita and Reed, 1995). Initiated cancer
cells may lead to tumor development only when a
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dysfunction in their apoptotic pathway takes place; some
of the mechanisms leading to inactivation of the apoptotic
pathway in cancer cells may result from an up-regulation
in the bcl-2 gene (a Bcl-2 chimeric factor is produced in
leukemias as a result of a translocation) or downregulation
of the bax gene. Gene therapy for cancer could
involve restoration of the apoptotic pathway in cancer
cells leading to their suicidal death (see below).
Figure 21. Involvement of Bax and Bcl-2 proteins in
apoptosis. Bax is a potent inducer of apoptosis; binding of Bcl-2
to Bax (also binding of the E1B 19 kDa protein of adenovirus to
Bax) prevents Bax from its apoptotic functions. From Boulikas T
(1997) Gene therapy of prostate cancer: p53, suicidal genes, and
other targets. Anticancer Res 17, 1471-1506. With the kind
permission of Anticancer Research.
Binding sites for p53 have been found at the origin of
replication of polyomavirus with an inhibitory effect on
virus replication in vitro (Miller et al, 1995) and at the
SV40 ORI (Bargonetti et al, 1991) as well as in putative
cellular origins of replication (Kern et al, 1991).
A number of genes not containing p53 response
elements may be repressed by p53 (Ginsberg et al, 1991;
Mercer et al, 1991; Shiio et al, 1992; Seto et al, 1992).